As a selective inhibitor of P-gp, LY335979 can potently inhibit P-gp with Ki of 59 nM, but does not modulate MRP-mediated resistance by MRP1 (ABCC1) and MRP2 (ABCC2). LY335979 is 500-1500 times more potent than cyclosporin A or verapamil in restoring Pgp substrate accumulation in the MDR cell line HL60/VCR. LY335979 could effectively block Pgp function on isolated CD56(+) lymphocytes and CD56(+) lymphocytes in whole blood with IC50 of 1.2 nM and 174 nM.